1. Field of the Invention
The present invention relates to an image detecting device for converting light into an electric signal and obtaining an image, in particular, to an image detecting device for directly converting light into electric charge and obtaining an image. In addition, the present invention relates to an image detecting device for use with a medical X-ray diagnosing system. Moreover, the present invention relates to a bi-plane type image detecting device having a plurality of image detecting planes.
2. Description of the Related Art
Image detecting devices with photoelectric elements as image detecting elements have been widely used for video cameras, digital still cameras, and so forth. In addition, such image detecting devices instead of conventional silver-halide films have been used for medical X-ray diagnosing system.
In recent years, databases for medical data of patients have been created so as to promptly and accurately treat the patients. There are also needs of databases for X-ray image data. Thus, it is desired to digitize detected X-ray images.
In medical X-ray diagnosing systems, diagnosis images have been photographed with silver-halide films. To digitize such images, after a photographed film is developed, the developed image should be scanned with a scanner. However, digitized images cannot be quickly and easily obtained. In addition, when the developed images are scanned, the image quality deteriorates.
Recently, a system that directly detects digital images with a CCD camera whose diameter is as small as for example one inch has been accomplished.
However, when the lungs of a patient are detected, it is necessary to detect an area of around 30 cm.times.30 cm. Thus, since an optical device that collects light is required, the size of the image detecting device becomes large.
As a system for solving such a problem, an image detecting device using thin film transistors (TFTs) composed of a-Si (amorphous silicon) as a semiconductor film has been proposed (for example U.S. Pat. No. 4,689,487).
FIG. 15 is a block diagram showing an example of the structure of such an image detecting device. FIG. 16 is a schematic diagram showing an example of the structure of an image detecting device using a-Si TFTs.
An X-ray radiated from an X-ray source 51 penetrates an object 52 (a human body) and enters photoelectric elements of the a-Si TFT image detecting device 53. The a-Si TFT image detecting device 53 converts the X-ray that has penetrated the object 52 into an analog signal corresponding to the dose of the X-ray. An A/D converting portion 57 converts the analog signal into a digital signal in time series. The digital signal is stored in an image memory 58.
The image memory 58 can store image data for one to several pictures. The image data is successively stored to predetermined addresses corresponding to a control signal received from a controlling portion 63. An arithmetic processing portion 59 extracts image data from the image memory 58, calculates the extracted image data, and stores the calculated result to the image memory 58. The calculated image data is supplied from the image memory 58 to a D/A converting portion 60. The D/A converting portion 60 converts the digital signal into an analog signal. The analog signal is output to an external processing circuit such as an image monitor 61 through an interface. Then the X-ray transmitted images of the object 52 are displayed on the image monitor 61.
In FIG. 16, a pixel e1,l that is an element of an image detecting area is composed of an a-Si thin film transistor 144, a photoelectric film 140, and a pixel capacitor 142. The thin film transistor 144 is composed of a-Si as a semiconductor film. Pixels are arranged in a matrix array of 2000 (W) pixels.times.2000 (L) pixels (hereinafter referred to as thin film transistor array).
A bias voltage of a power supply 148 is applied to the photoelectric film 140. The a-Si TFT 144 is connected to a signal line S1 and a scan line G1. The a-Si TFT 144 is turned on/off corresponding to the voltage of the scanning signal applied from a scan line driving circuit 152 to the gate electrode through the scan line. A first end of the signal line S1 is connected to an amplifier 145 such as a sense amplifier that detects a signal.
When light (for example, an X-ray or a soft X-ray) enters the photoelectric film, a current flows in the photoelectric film 140. Thus, electric charge is stored in the pixel capacitor 142. When the scan line driving circuit 152 drives the scan line and thereby all TFTs connected thereto are turned on, the stored electric charge flows to the amplifier 154 through the signal line S1. Due to the difference of the amount of electric charge corresponding to the dose of light entered into each pixel, the amplitude of an output signal of the amplifier 154 varies corresponding to the difference. In the device shown in FIG. 16, when an analog signal that is output form the amplifier 154 is converted into a digital signal, a digital image can be directly obtained.
The structure of the pixel area shown in FIG. 16 is the same as the structure of a TFT-LCD that is used for a note type personal computer. The TFT-LCD is an active matrix type liquid crystal display of which thin film transistors are used as switching elements. Thus, an image detecting device that is thin and that has a large screen can be easily fabricated.
There are two types of image detecting devices that convert light such as an X-ray into electric charge.
The first type is referred to as indirect converting type. In the indirect converting type, an X-ray is converted into visible light by phosphor, scintillator or the like. The visible light is converted into electric charge by a photoelectric film. The second type is referred to as direct converting type. In the direct converting type, an X-ray is directly converted into electric charge by a photoelectric film.
In the indirect converting type, although an X-ray is converted into visible light by scintillator, since the light scatters in the scintillator, a sufficient resolution cannot be obtained. On the other hand, in the direct converting type, since an X-ray does not penetrate, an image with a high resolution can be obtained. An image with a high resolution is an essential condition for a medical image. Thus, a direct converting type image detecting device is becoming attractive.
However, in the direct converting type image detecting device, the photoelectric film should be thickened (for example, in the range from several 100 .mu.m to several mm) so as to improve the efficiency for converting an X-ray into electric charge. Thus, to apply a proper electric field to the photoelectric film, a high voltage of several kilovolts should be applied to the photoelectric film. Thus, in the direct converting type image detecting device, it is necessary to take countermeasures against dielectric breakdown of the TFT array.
As one of the countermeasures against dielectric breakdown, a method for forming a dielectric layer on a photoelectric film has been proposed (as U.S. Pat. No. 5,319,206). However, in this method, electric charge stored in the dielectric layer cannot be quickly discharged. Thus, images cannot be successively detected.
To prevent dielectric breakdown from taking place in a TFT array, a method for disposing a protecting circuit in a pixel e (i, j) has been disclosed (for example, Japanese Patent Laid-Open Application Nos. 10-10237 and 10-170658).
FIG. 24 shows an example of an a-Si TFT image detecting device of which each pixel has a high voltage protecting diode.
In FIG. 24, each of pixels e (i, j) is composed of an electric charge reading a-Si TFT 201, an electric charge capacitor 202, a protecting diode 214, and a photoelectric film 203. The pixels e (i, j) are arranged in a matrix array of 2000 (W) pixels.times.2000 (L) pixels (hereinafter referred to as thin film transistor array). A several kV high voltage of a power supply 205 is applied to a photoelectric film 203. The source electrode of the a-Si TFT 201 is connected to a signal line S1. The gate electrode is connected to a scan line G1. The protecting diode 214 and the electric charge capacitor 202 are connected in parallel. The first end of the protecting diode 214 is connected to a bias line Bias. Normally, a voltage is applied to the bias line Bias so that the protecting diode 214 is turned off. When an X-ray enters the a-Si TFT image detecting device, a current flows in the photoelectric film 203. Thus, the voltage of a pixel electrode (a connected portion (P) of an electric charge reading a-Si TFT 201 and an electric charge capacitor 202) rises. When an X-ray excessively enters the image detecting device as if an X-ray directly enters the image detecting device not through the object), in the worst case, the voltage of the pixel electrode rises to the power supply voltage (several kV) supplied to the photoelectric film 203. thus, when the voltage of the pixel electrode exceeds a predetermined voltage, the protecting diode 214 operates so that excessive electric charge flows to the bias line Bias. Since the voltage of the pixel electrode is limited to a predetermined voltage, dielectric breakdown can be prevented from taking in the insulating film.
In an image detecting device such as an X-ray image detecting device, image detecting modes such as penetration, DA (Digital-Angiography), and DSA (Digital-Subtraction-Angiography) are used. The penetration mode is a moving picture mode. The DA and DSA modes are still picture modes. The dose of an X-ray and a frame rate of a detected image vary corresponding to each image detecting mode.
In the moving picture mode, the dose of a radiated X-ray is around several .mu.R/frame and the frame rate of a detected image is in the range from around 30 to 100 fps. On the other hand, in the still picture mode, the dose of a radiated X-ray is around several 100 .mu.R/frame and the frame rate of a detected image is several fps.
In the method of which a protecting circuit is disposed in each pixel, an image can be detected in any image detecting mode of moving picture mode and still picture mode.
However, the amount of electric charge stored in each pixel in the moving picture mode is different from that in the still picture mode by around three digits. Thus, the performance required for the image detecting device depends on the image detecting mode. In other words, in the moving picture mode (with a high frame rate), low nose is the most important factor. On the other hand, in the still picture mode, a large signal is the most important factor.
The protecting diode 214 is normally turned off. Even in the off state, a very weak leak current (around 10.sup.-10 to 10.sup.-14 [A]) always flows. This current is stored in the pixel. Thus, image quality deteriorates such as low dynamic range and large noise.
In the penetration mode, since the signal level of the current stored in the pixel is low, the influence of the leak current is relatively large.
In addition, as described above, the amount of electric charge stored in the pixel in the penetration mode is different from that in the DSA mode by around three digits. Thus, the dynamic range of the A/D converter does not sufficiently correspond to the dynamic range of stored electric charge.
The conventional X-ray image detecting devices have been for a chest diagnosing system, a bust diagnosing system, and a circulatory organ diagnosing system. In these systems, X-ray image detecting devices with a large screen, a high accuracy, and a high frame rate are required.
For example, in the chest diagnosing system, an image detecting area of around 300.times.300 mm is required. In the bust diagnosing system, a pixel size of 50 .mu.m square is required. In the circulatory organ diagnosing system, a frame rate in the range from around 50 to 100 frames/second is required.
However, as shown in FIG. 17, each scan line has a line resistance. In addition, line capacitors are formed at the intersection of each scan line and each signal line and between the gate electrode and the source electrode of each TFT. Thus, signal delays and waveform distortion take place in a scan line driving signal. Due to the signal delays and waveform distortion, a selection period and a gate voltage necessary for turning on each TFT cannot be obtained. Thus, since electric charge stored in each pixel cannot be sufficiently read out, an image detecting device with a large screen, a high resolution, and a high frame rate cannot be accomplished.
In particular, when an object is detected in the moving picture mode, an image detecting device with a high resolution and a high frame rate should be accomplished. Thus, the development of an image detecting device with a high resolution and a high frame rate is required.